The present invention relates to an arrangement for actuating at least first and second mechanically independent valves in a hydraulic system and for providing a pressure fluid interlock to ensure that one of the valves is fully closed before the other is opened, the arrangement comprising first and second pressure-fluid piston-cylinder actuator units for actuating the first and second valves of the pressure fluid system and first and second electromagnetically-actuated pilot valves for controlling the operation of the first and second piston-cylinder units. The invention also relates to a hydraulic system comprising the first and second valves and the arrangement of the invention, and also a machine, such as for instance a hydraulic press, comprising a hydraulic piston-cylinder unit controlled by the system of the invention.
There are hydraulic systems in which water is used as the pressure fluid and in which the necessary interlock is provided by control devices which can act in various ways, namely mechanically, or electro-hydraulically using the hydraulic fluid which is also used for the hydraulic piston-cylinder units, or electro-hydraulically using a different hydraulic fluid or electro-pneumatically using compressed air.
In general, the hydraulic systems with which the invention is concerned will be systems in which hydraulic pressure can be obtained either from a direct pump connection of from one or more pressure accumulators.
To control the hydraulic piston-cylinder unit in a machine such as a hydraulic press, at least two valves are required in practice, and for practical reasons, these valves are normally mechanically independent, but their opening and closing movements are coordinated or interlocked. Interlocking is important, particularly when using a pressure accumulator; in this case, the discharge valve must be closed before the inlet valve is opened, and the inlet valve must be closed before the discharge valve is opened, in order to prevent any loss of hydraulic fluid during change-over. If this interlock procedure is not followed, both valves are open at the same during the change-over procedure. The hydraulic fluid then flows direct from the inlet valve to the discharge valve and is not used, resulting to a substantial loss of energy during successive change-over operations.
German Patent Specification No. 1 269 457 and German Offenlegungsschrift No. 1 402 525 disclose control arrangements for providing the interlock of mechanically-independent valves in servomotors. The interlock of the first and second piston-cylinder actuator units is effected by providing slot or port control of the pistons of the units. With this type of control, even if hydraulic control pulses are transmitted simultaneously to the pilot valves via the slots or ports, the piston-cylinder units are controlled in such a way that one valve cannot be opened until the other valve has been closed.
In general, this method of interlock by slot or port control is very reliable in practice. However, a succession of faulty electrical control signals to the pilot valves can destroy the interlock effect. For instance, if an incorrect electrical signal to open the discharge valve is given while the inlet valve is still open, oil will leak from one port of the piston-cylinder unit actuating the inlet valve to the other port, between the cylinder wall and the piston. Since the pilot valve controlling the discharge valve has been actuated magnetically to open the discharge valve, the oil leaking from the piston of the inlet valve can now gradually pass through the open pilot valve to the associated piston which actuates the discharge valve, and cause the latter piston to open the discharge valve. In this way, the discharge valve is opened, although the inlet valve has not completely closed.